Turbine rotors



Oct. 5, 1965 A. FRANKEL 3,209,838

TURBINE ROTORS Filed July 29, 1963 2 Sheets-Sheet l A. FRANKEL TURBINE ROTORS Oct. 5, 1965 2 Sheets-Sheet 2 Filed July 29, 1963 United States Patent 3,209,838 TURBINE ROTORS Adolf Frankel, Altrincham, England, assiguor to Associated Electrical Industries Limited, London, England, a British company Filed July 29, 1963, Ser. No. 298,251 Claims priority, application Great Britain, Aug. 22, 1962, 32,285/ 62 3 Claims. (Cl. 170-169) This invention relates to improvements in turbine rotors and bladed discs or wheels therefor, and relates more particularly to the provision of improved bracing means for the blades of such rotor discs and wheels.

A large steam turbine includes two or more cylinders in each of which the turbine rotor is provided with circumferential series of rotor blades, each series comprising a stage of the turbine. It is important that the steam passing between the blades of a rotor stage shall not escape about the periphery of blades of the stage, and a shroud ring is therefore normally fitted to limit the steam flow to the spaces between the blades. For the high pressure and intermediate pressure cylinders of a turbine, a sectionalized frusto-conical shroud ring is used, each section being joined to a group of blades, say seven in number, and there being a clearance between the butting ends of adjacent sections to permit thermal expansion. These shroud ring sections also serve to brace the blades, so reducing vibration of the blades and the danger of blade failure due to such vibration.

The normal frusto-conical type of shroud suffers, particularly when applied to stages having blades with a very large outside diameter running at a high peripheral speed, as in the last stages of the low pressure cylinder, from the disadvantage that the effective attachment of the shroud sections to the blades by the conventional rivetting over of projections on the blades becomes diificult as result of the large centrifugal forces due to the high peripheral speeds, and from the disadvantage that the bending stresses in the shroud material become very large due to the effect of centrifugal force combined with the increasing span which has to be bridged between adjoining blades.

In the case of the last stages of the low pressure cylinder it is normally found impracticable to use a continuous frusto-conical or cylindrical shrouding ring or band, although this would be highly advantageous from the point of view of protecting the blade row against failure. This difficulty is due to the fact that the distance between adjoining blade tips increases in operation due to the radial stretch which the blades and the disc on which the blades are mounted suffer under the action of centrifugal force. Since the shrouding ring can be caused to accommodate this change in blade tip pitch only by producing excessive stresses in the system, it has been found necessary to introduce some form of sectionalised shroud ring. Each section of shroud ring keeps the blade tip pitch of the blades which it serves at a substantially constant value, so that the outermost blades of the group of blades must deflect circumferentially. The deflections are progressively larger for the blades towards each end of the group.

An object of the present invention is the provision of an improved turbine rotor provided with blade bracing means in which the disadvantages discussed above are at least mitigated.

According to the present invention, a turbine rotor includes at least one circle of rotor blades which are joined by bracing ring sections each extending between and attached to two adjacent blades and each curved, as viewed in a direction parallel to the rotor axis, outwardly beyond the circle including the parts of the sections which lie 3,209,838 Patented Oct. 5, 1965 "ice adjacent the points of attachment of the sectidns to the blades.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGURE 1 is'a sectional side elevation of one bladed wheel or disc mounted on the rotor of a steam turbine;

FIGURE 2 is a view taken in a direction parallel to the rotor axis of FIGURE 1, and shows a small part only of the periphery of the bladed wheel or disc;

FIGURE 3 illustrates a second embodiment of the invention and is a sectional drawing taken on a plane normal to the direction of steam flow between two adjacent blades of a turbine wheel or disc;

FIGURE 4 illustrates an alternative embodiment of the invention and corresponds to FIGURE 2; and

FIGURE 5 is a view in the radially inwards direction of the construction shown in FIGURE 3.

Referring first to FIGURES 1 and 2, the turbine rotor shaft 1 has secured to it a large number of bladed wheels or discs, of which one wheel 3 is shown. Wheel 3 con sists of a hub portion 3H and a thinner disc portion 3D the periphery of which is machined to accommodate in known manner the shaped root portions of blades 5. As shown in FIGURE 2, the tip of each blade is thickened at 5A and on each axially extending side is formed with an axially extending undercut groove 53. Between each pair of adjacent blades, a shroud ring section 7 is fitted, comprising a central portion 7A which is curved, as viewed in a direction parallel to the rotor axis, outwardly beyond the circle R including the points of attachment of the ring sections 7. The ends 7B of the sections 7 are enlarged and are complementary to the grooves 5B, so that the section 7 can be slid into place in the grooves and is then effective to locate each blade tip relative to the adjacent blade tip in a circumferential direction.

The curvature of the portion 7A is that of a catenary, and the design of the section 7 will be clearer after a consideration of the improved manner in which this shroud ring section operates.

In use of the turbine at its design or optimum speed, the centrifugal force effect will cause a percentage elongation of the blade pitch, e, which can be computed by well established methods from the calculated stress distribution in the turbine disc and the turbine blades. Also, since the material of the shroud ring section is subjected to centrifugal force, a radially directed outward force will be produced on each particle of the section 7. As a result, if one considers any intermediate part of the section 7, it is clear that that intermediate part will be put in a state of tension by the centrifugal force, and will be caused to elongate. Ideally, the total elongation produced in the shroud ring section by the centrifugal force should correspond to the elongation of the blade tip pitch by the centrifugal force acting on the blades and turbine disc, although in practice a close approximation can produce much of the advantages of the present invention.

As regards the design of the catenary shape, the tensile stress f which is required in the catenary to obtain the percentage elongation of e in the turbine blade pitch, at the diameter at which the shroud is attached, is given y where E is the modulus of elasticity of the shroud material. The total tensile pull P through the catenary is given by P=f.A

where A is the cross-section of the catenary, and this must have a radial component W/ 2 where W is the total centrifugal pull on the shroud. This condition must be met if a geometry free of bending stress is to be produced as desired. It follows that in s a- P where a is the angle between the chord and the tangent to the catenary at the point of attachment. The shape of the catenary is defined by conventional mathematical methods by this angle and theknown pitch between the points of attachment.

The centrifugal pull on the section 7A, which is equal to W/2 at each end of the section 7A, must be taken by its attachments to the blade tips. FIGURE 2 shows one way in which this pull can be transmitted to the blade tips.

In some cases it will be desirable to provide support between the rotor blades at a point intermediate the point of attachment to the hub portion 3H and the blade tips, and a supporting band can be provided consisting of sections formed similar to the shroud sections described above.

FIGURE 3 illustrates banding sections 27 each having a curved portion 27A and radial end parts 27E which are secured by rivets 29 to the blades at a point intermediate their length. In this case, the curvature of the portions 27A is decided in the manner described above by the elongation of the pitch distance of the blades 5 at the points of attachment of the sections 27 to the blades. The method of attachment of the sections 27 to the blades shown in FIGURE 3 can be used also for the attachment of peripheral shroud sections to the blade tips.

Thus in FIGURES 4 and 5 each banding section 37 includes a curved portion 37A and radial end parts 37E which are secured by rivets 39 extending through the blades 5 adjacent their radially outer ends to the adjacent banding sections. The rivets 39 extend perpendicularly to the blade section, so that they take the tension force of the shroud partly in tension and partly in shear, but mainly in shear.

It will be clear from FIGURE 5 that in this arrangement the complete shroud ring assembly is made up of two rings lying side-by-side, as indicated by ring A and ring B in FIGURE 5. Detailed stress investigation shows that a shroud of rhomboidal shape (as seen in radial view, e.g. as in FIGURE 5) is subject to some shear stresses. The shear stresses increase as the rhombus departs more and more from the rectangular shape, and as its width increases. The inclination of the short faces of the rhombus is determined by the geometry of the blades it is intended to use, and therefore the shear stresses are suitably limited by splitting the shroud ring assembly as shown in rings A and B.

If desired, the shroud ring assembly can be split into several rings lying side-by-side.

What I claim is:

1. A turbine rotor comprising at least one rotor disc; a circumferential row of rotor blades on the disc; socket means formed at the tip of each rotor blade; bracing ring sections joining adjacent blades in the circumferential row adjacent the tip of said blade; each bracing riing section extending between two adjacent blades in the row; each bracing ring section having enlarged portions at each end thereof, said portions being secured in said socket means for attaching said bracing ring section to the rotor blades; and each bracing ring section including a curved part, curved, as viewed in a direction parallel to the rotor axis, so that a central region of the curved part is at a greater radius than two flanking regions of the curved part which are respectively adjacent to the two blades; the curved part being curved to a catenary curve having its ends inclined at an angle satisfying the equation where a is the angle between the chord joining the two points of attachment and the tangent to the catenary at each of these points; W is the total centrifugal pull on the bracing ring sections; e is the percentage elongation of the turbine blade pitch in use of the rotor at its optimum speed; E is Youngs modulus of elasticity of the material of the bracing ring section; and A is the area of the cross section of the bracing ring section; whereby in use of the rotor at its optimum speed, the percentage elongation of the curved part due to centrifugal force action is substantially equal to the percentage increase in pitch between adjoining blades which is due to centrifugal force on the blades, so that the sections accommodate the effect of centrifugal force on the rotor without appreciable change in their curvature.

2. A rotor according to claim 1, in which a plurality of bracing ring sections are spaced inwardly of said first mentioned bracing ring sections and are connected between adjacent rotor blades by pin-like members which extend perpendicularly through the blade between the two sections and transmit tension forces from one ring section to the other.

3. A rotor according to claim 2, in which the first mentioned bracing ring sections are substantially rhomboidal as viewed in a radial direction, and the first mentioned bracing ring sections and the second mentioned bracing ring sections are arranged in a plurality of concentric circles, lying side by side.

sin a:

References Cited by the Examiner UNITED STATES PATENTS 1,499,103 6/24 Flanders 253-772 2,914,299 11/59 Mitchell 253-77.2

FOREIGN PATENTS 563,458 11/32 Germany. 315,722 2/30 Great Britain.

SAMUEL LEVINE, Primary Examiner. JULIUS E. WEST, Examiner. 

1. A TURBINE ROTOR COMPRISING AT LEAST ONE ROTOR DISC; A CIRCUMFERENTIAL ROW OF ROTOR BLADES ON THE DISC; SOCKET MEANS FORMED AT THE TIP OF EACH ROTOR BLADE; BRACING RING SECTIONS JOINING ADJACENT BLADES IN THE CIRCUMFERENTIAL ROW ADJACENT THE TIP OF SAID BLADE; EACH BRACING RING SECTION EXTENDING BETWEEN TWO ADJACENT BLADES IN THE ROW; EACH BRACING RING SECTION HAVING ENLARGED PORTIONS AT EACH END THEREOF, SAID PORTIONS BEING SECURED IN SAID SOCKET MEANS FOR ATTACHING SAID BRACING RING SECTION TO THE ROTOR BLADES; AND EACH BRACING RING SECTION INCLUDING A CURVED PART, CURVED, AS VIEWED IN A DIRECTION PARALLEL TO THE ROTOR AXIS, SO THAT A CENTRAL REGION OF THE CURVED PART IS AT A GREATER RADIUS THAN TWO FLANKING REGIONS OF THE CURVED PART WHICH ARE RESPECTIVELY ADJACENT TO THE TWO BLADES; THE CURVES PART BEING CURVED TO A CATENARY CURVE HAVING ITS ENDS INCLINED AT AN ANGLE SATISFYING THE EQUATION 